Re: [PATCH] ima: setting security.ima to fix security.evm for a file with IMA signature

[Mimi Zohar <zohar@linux.ibm.com>]

On Tue, 2025-09-09 at 12:19 +0800, Coiby Xu wrote:
> When both IMA and EVM fix modes are enabled, accessing a file with IMA
> signature won't cause security.evm to be fixed. But this doesn't happen
> to a file with correct IMA hash already set because accessing it will
> cause setting security.ima again which triggers fixing security.evm
> thanks to security_inode_post_setxattr->evm_update_evmxattr.
> 
> Let's use the same mechanism to fix security.evm for a file with IMA
> signature.
> 
> Signed-off-by: Coiby Xu <coxu@redhat.com>

Agreed, re-writing the file signature stored as security.ima would force
security.evm to be updated.

Unfortunately, I'm missing something. ima_appraise_measurement() first verifies
the existing security.evm xattr, before verifying the security.ima xattr.  If
the EVM HMAC fails to verify, it immediately exits ima_appraise_measurement(). 
security.ima in this case is never verified.

This patch seems to address the case where the existing security.evm is valid,
but the file signature stored in security.ima is invalid.  (To get to the new
code, the "status" flag is not INTEGRITY_PASS.)  Re-writing the same invalid
file signature would solve an invalid security.evm, but not an invalid IMA file
signature.  What am I missing?

thanks,

Mimi

> ---
>  security/integrity/ima/ima_appraise.c | 27 +++++++++++++++++++++------
>  1 file changed, 21 insertions(+), 6 deletions(-)
> 
> diff --git a/security/integrity/ima/ima_appraise.c b/security/integrity/ima/ima_appraise.c
> index f435eff4667f..18c3907c5e44 100644
> --- a/security/integrity/ima/ima_appraise.c
> +++ b/security/integrity/ima/ima_appraise.c
> @@ -595,12 +595,27 @@ int ima_appraise_measurement(enum ima_hooks func, struct ima_iint_cache *iint,
>  		integrity_audit_msg(audit_msgno, inode, filename,
>  				    op, cause, rc, 0);
>  	} else if (status != INTEGRITY_PASS) {
> -		/* Fix mode, but don't replace file signatures. */
> -		if ((ima_appraise & IMA_APPRAISE_FIX) && !try_modsig &&
> -		    (!xattr_value ||
> -		     xattr_value->type != EVM_IMA_XATTR_DIGSIG)) {
> -			if (!ima_fix_xattr(dentry, iint))
> -				status = INTEGRITY_PASS;
> +		/*
> +		 * Fix mode, but don't replace file signatures.
> +		 *
> +		 * When EVM fix mode is also enabled, security.evm will be
> +		 * fixed automatically when security.ima is set because of
> +		 * security_inode_post_setxattr->evm_update_evmxattr.
> +		 */
> +		if ((ima_appraise & IMA_APPRAISE_FIX) && !try_modsig) {
> +			if (!xattr_value ||
> +			    xattr_value->type != EVM_IMA_XATTR_DIGSIG) {
> +				if (ima_fix_xattr(dentry, iint))
> +					status = INTEGRITY_PASS;
> +			} else if (xattr_value->type == EVM_IMA_XATTR_DIGSIG &&
> +				   evm_revalidate_status(XATTR_NAME_IMA)) {
> +				if (!__vfs_setxattr_noperm(&nop_mnt_idmap,
> +							   dentry,
> +							   XATTR_NAME_IMA,
> +							   xattr_value,
> +							   xattr_len, 0))
> +					status = INTEGRITY_PASS;
> +			}
>  		}
>  
>  		/*
> 
> base-commit: b320789d6883cc00ac78ce83bccbfe7ed58afcf0